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1.
Vale FR  Jackson WA  Volk RJ 《Plant physiology》1987,84(4):1416-1420
Potassium influx into roots of dark-grown decapitated maize seedling (Zea mays L., cv Pioneer 3369A) was examined in presence and absence of ambient ammonium and at various root potassium concentrations. Six-day old seedlings which were dependent on the endosperm reserves for their energy source were exposed to KCl (labeled with 86Rb) ranging from 5 to 200 micromolar. At both low (13 micromoles per gram fresh weight) and high (100 micromoles per gram fresh weight) root potassium concentration, isotherms indicated two potassium influx systems, one approaching saturation at 50 to 100 micromolar potassium and an additional one tentatively considered to be linear. A mixed-type inhibition by ammonium for the low-concentration saturable system was indicated by a concomitant decrease in Vmax and increase in Km. High root potassium concentration decreased Vmax but had little effect on Km of this system. The rate constant for the second quasilinear system was decreased by ambient ammonium and by high root potassium status. Transfer of high potassium roots to potassium-free solutions resulted in an increase in influx within 2 hours; by 24 hours influx significantly exceeded that of roots not previously exposed to potassium. In roots of both low and high root potassium concentrations, potassium influx was restricted progressively as ambient ammonium increased to about 100 micromolar, but there was little further inhibition as ammonium concentrations increased beyond that to 500 micromolar. The data imply that potassium influx has two components, one subject to inhibition by ambient ammonium and one relatively resistant.  相似文献   

2.
Seedlings of barley (Hordeum vulgare L. cvs Salka and Zita), buckwheat (Fagopyrum esculentum Moench) and rape (Brassica napus L. ssp. napus cv. Line) were raised at 8 or 10 different extenral P concentrations in the range 0–2000 μM. Apart from P, the nutrient solutions were complete. Phosphate influx in roots of different P status was determined by use of a nutrient solution containing 0.1 mM32P-labelled phosphate. A double labelling technique was used for simultaneous determination of influx (33P) and efflux (32P) of phosphorus by roots of barley and rape with three selected P levels. Flux determinations were also done in presence of a metabolic uncoupler (2,4-dinitrophenol) and a protein synthesis inhibitor (cycloheximide). Influx of phosphate was maximal at a certin optimal P level of the roots and decreased at both lower and higher P levels. Maximum phosphate influex [μmol (g root)-?1 h?1] were: rape 4,4, buckwheat 2.2, barley cv. Salka 1.6, barley cv. Zita 1.5. Both Hill plots and plots of the untransformed decreasing phosphate influx vs root P concentrations above the optimal were linear and had high correlation coefficients. The Hill coefficient varied between -3.1 and -4.2. The decrease of phosphate influx from the maximum to the lowest value at the highest P concentration of the root was 60–70%. Hence, phosphate influex appeared to be regulated through negative feedback by the internal level of phosphorous in the roots. The regulation mechanism seems bascially similar for the three species and may be of an allosteric type. P efflux from roots of low and optimal (with regard to P influx) P status was 15–20% of the simultaneous P influx. Contary to P influx, P efflux increased at high P status and almost eliminated (barley) or halved (rape) net P uptake. 2,4-Dinitrophenol reduced both P influx and P efflux by low P roots and gave linearly increasing P efflux with increasing root P status. This indicates that P efflux partly occurred by counter transport and ion exchange at the uptake sites, partly by passive P efflux along an electrochemical potential gradient. Phosphate influx was not affected by inhibition of barley root growth with cycloheximide, but P efflux increased considerably.  相似文献   

3.
Wheat seedlings (Triticum aestivum L. cv Gamenya) were grown for 4 days in culture solutions of differing pH prior to studying their subsequent short-term absorption of 32Pi from solutions of the same or different pH.

Increasing pH of the absorption solution from 5.5 to 7.0 or 8.0 depressed 32Pi absorption from 1 and 10 micromolar Pi but had little effect at 100 and 1000 micromolar Pi. Increasing the pH of the culture solution from 4.5 to 6.5 doubled or trebled subsequent 32Pi absorption from nearly all absorption solutions over a wide range of Pi concentrations, pH, and nutrient compositions.

When seedlings were transferred between culture pH treatments 4.5 and 6.5, their capacity for 32Pi absorption remained unchanged for at least 5 hours and adjusted by 60 to 80% after 24 hours and completely after 48 hours. This suggests that the root's capacity to absorb Pi responds to pH through slow structural changes in its mechanism of Pi absorption. P content and concentration of wheat seedlings reflected the response of 32Pi absorption to culture pH.

It is suggested that absorption pH affects an activity component of the process for Pi absorption and culture pH affects a capacity component. Failure to recognize the capacity component of the pH response explains why previously published results for short-term 32Pi absorption conflict with those for long-term P accumulation in plants.

  相似文献   

4.
M. A. Topa 《Plant and Soil》1996,182(2):259-265
Short-term 32P uptake experiments were conducted with intact seedlings of loblolly pine (Pinus taeda L.) to examine possible seed source variation in net accumulation of 32P in roots and shoots, and in rates of unidirectional influx. Seed source had a highly significant effect on biomass and P concentrations of shoots and roots. Seedlings from two seed sources representing fast-growing populations (a broadly-adapted and wet-site seed source) accumulated over 60% more total seedling P than smaller seedlings from a drought-hardy seed source, reflecting higher biomass and root P concentrations. Rates of unidirectional 32P influx in seedlings from the drought-hardy seed source were more than twice the rates of the seedlings from the broadly-adapted seed source. However, after 24 h in labeled uptake solution, net accumulation of 32P was similar, suggesting that rates of unidirectional efflux from roots of the drought-hardy seed source were also high. Although there were no significant differences in biomass and tissue P concentrations between the two fast-growing seed sources, rates of unidirectional influx in seedlings from the broadly-adapted seed source were 42% lower than rates in seedlings from the wet-site source. Yet, after 24 h in labeled uptake solution, net accumulation of 32P in seedlings from the broadly-adapted seed source was 50% higher. Unidirectional efflux out of the root may regulate net uptake of P as much, if not more, than influx in loblolly pine seedlings-at least under high-P growth conditions. The results in this study do not support previous studies with herbaceous plants suggesting that fast-growing species typically exhibit higher rates of nutrient uptake than slow-growing species.  相似文献   

5.
We examined the effects o long-term hypoxic growth conditions on net uptake and transport of P to shoots of pond Pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine. Seedlings were grown under aerobic orhypoxic solution conditions for 4–5 weeks in continuously flowing solution culture containing 100 μM P. Short – and long-term 32P. experiments were then concluded with intact seedlings to determine rates of 32P influx, efflux and net transport to the shoot. Shoot fresh weight/root fresh weight ratios were significantly higher under hypoxic gorwth conditions, reflecting the larger reduction in root growth than shoot growth, despite extensive aerechyma formation in roots. Estimates for the unidirectional influx of 32P in aerobic and hypoxic seedlings were 1.43 and 3.20 μmol P (gFW root)?1 h?1, respectively. However, 32P accumulation between the two treatments became similar within 8 h, suggesting that efflux was also higer in seedlings from the hypoxic treatment. Indeed in a separate experiment, hypoxic growth conditions increased efflux by over 60%. Transport of 32P to shoots was significantly reduced under hypoxic growth conditions, despite higher root P concentrations and lower shoot P concentrations. After 48 h, 32P accumulation in roots was similar between the two treatments. Yet total accumulation of seedling 32P decrcased by 31% under the hypoxic treatment, largely because of reduced transport of 32p to the shoot. The lower accumulation of 32 by shoots of seedlings in the hypoxic treatment may be the result of a direct inhibition on the transport process in O2-defident tissues, but could also reflect a slower turnover or labeling of the ool available for transport. Indeed, the percentage of total 32P in. roots present in the soluble P. (or transportable form of P) was about 33% lower in seedlings from the hypoxic treatment, probably reflecting increased assimilation into organic compounds as well as chelation with iron. Our results suggest that P transport to the shoots of acclimated seedlings may be more sensitive to hypoxic solution conditions than influx at the root Plasmalemma.  相似文献   

6.
Barley (Hordeum vulgare L. cv Halcyon) seedlings which had been grown in full strength complete inorganic nutrient media (containing 6 millimolar K+) had high internal K+ concentrations and low values of K+ (86Rb+) influx when influx was measured from solutions containing 100 micromolar K+. Transfer of these plants to solutions lacking K+ resulted in significant reductions of root and shoot K+ concentrations and values of K+ (86Rb+) influx increased by greater than 10-fold within 3 days. When plants treated in this way were returned to complete solutions, containing K+, the changes induced by K+ deprivation were reversed. Parallel studies of microsomal membranes by means of SDS-PAGE demonstrated that the expression of a group of polypeptides increased or decreased in parallel with changes of K+ (86Rb+) influx. Most prominent of these were 45 and 34 kilodalton polypeptides which specifically responded to K+ status of the barley plants; their expression was not enhanced by N or P deprivation. The 45 kilodalton polypeptide was susceptible to degradation by a membrane associated protease when microsomes were washed in buffer containing 0.2 millimolar PMSF. This loss was prevented by increasing PMSF concentration to 2 millimolar.  相似文献   

7.
Uptake of glycine and release of primary amines by the polychaete Nereis virens (Sars) and the mud snail Hydrobia neglecta Muus was investigated. In a flow system, Nereis virens had a net influx of glycine of 32 to 39 nmol·g?1 fresh wt·h from a 5-μM solution of glycine. Simultaneously, an efflux of primary amines of 14 nmol·g?1·h?1 was measured. The experiments indicated that glycine uptake and release of primary amines are two independent processes. Glycine was absorved by Hydrobia neglecta from 50-μM solutions at an influx of 175 nmol·g?1 organic wt·h?1, and simultaneously, there was an independent efflux of 25 nmol primary amine·g?1·h?1. In the experiments, released primary amines from both species increased the ambient concentration of primary amines, but after various periods of time these concentrations apparently stabilized. Finally, the significance of micro-organisms in amino-acid uptake experiments is considered.  相似文献   

8.
The influence of NH4+, in the external medium, on fluxes of NO3 and K+ were investigated using barley (Hordeum vulgare cv Betzes) plants. NH4+ was without effect on NO3 (36ClO3) influx whereas inhibition of net uptake appeared to be a function of previous NO3 provision. Plants grown at 10 micromolar NO3 were sensitive to external NH4+ when uptake was measured in 100 micromolar NO3. By contrast, NO3 uptake (from 100 micromolar NO3) by plants previously grown at this concentration was not reduced by NH4+ treatment. Plants pretreated for 2 days with 5 millimolar NO3 showed net efflux of NO3 when roots were transferred to 100 micromolar NO3. This efflux was stimulated in the presence of NH4+. NH4+ also stimulated NO3 efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO3 occur via effects upon efflux. By contrast to the situation for NO3, net K+ uptake and influx of 36Rb+-labeled K+ was inhibited by NH4+ regardless of the nutrient history of the plants. Inhibition of net K+ uptake reached its maximum value within 2 minutes of NH4+ addition. It is concluded that the latter ion exerts a direct effect upon K+ influx.  相似文献   

9.
Experiments with intact plants of Lolium perenne previously grown with 14NO3 revealed significant efflux of this isotopic species when the plants were transferred to solutions of highly enriched 15NO3. The exuded 14NO3 was subsequently reabsorbed when the ambient solutions were not replaced. When they were frequently replaced, continual efflux of the 14NO3 was observed. Influx of 15NO3 was significantly greater than influx of 14NO3 from solutions of identical NO3 concentration. Transferring plants to 14NO3 solutions after a six-hour period in 15NO3 resulted in efflux of the latter. Presence of Mg2+, rather than Ca2+, in the ambient 15NO3 solution resulted in a decidedly increased rate of 14NO3 efflux and a slight but significant increase in 15NO3 influx. Accordingly, net NO3 influx was slightly depressed. A model in accordance with these observations is presented; its essential features include a passive bidirectional pathway, an active uptake mechanism, and a pathway for recycling of endogenous NO3 within unstirred layers from the passive pathway to the active uptake site.  相似文献   

10.
13N-labeled nitrate was used to trace short-term nitrate influx into Lemna gibba L. G3 in experiments where disappearance of both radioactivity and total nitrate from the incubation medium was measured continuously and simultaneously. In plants performing net nitrate uptake from an initial nitrate concentration of 40 to 60 micromolar, there was no discrepancy between net uptake and influx, irrespective of the N status of the plants, indicating that concomitant nitrate efflux was low or nil. Plants treated with tungstate to inactivate nitrate reductase were able to take up nitrate following induction of the uptake system by exposure to a low amount of nitrate. Also, in this case, net uptake was equivalent to influx. In tungstate-treated plants preloaded with nitrate, both net uptake and influx were nil. In contrast to these observations, a clear discrepancy between net uptake and influx was observed when the plants were incubated at an initial nitrate concentration of approximately 5 micromolar, where net uptake is low and eventually ceases. It is concluded that plasmalemma nitrate transport is essentially unidirectional in plants performing net uptake at a concentration of 40 to 60 micromolar, and that transport is nil when internal nitrate sinks (vacuole, metabolism) are eliminated. The efflux component becomes increasingly important when the external concentration approaches the threshold value for net nitrate uptake (the nitrate compensation point) where considerable exchange between internal and external nitrate occurs.  相似文献   

11.
13N-labeled nitrate was used to trace short-term nitrate influx into Lemna gibba L. G3 in experiments where disappearance of both radioactivity and total nitrate from the incubation medium was measured continuously and simultaneously. In plants performing net nitrate uptake from an initial nitrate concentration of 40 to 60 micromolar, there was no discrepancy between net uptake and influx, irrespective of the N status of the plants, indicating that concomitant nitrate efflux was low or nil. Plants treated with tungstate to inactivate nitrate reductase were able to take up nitrate following induction of the uptake system by exposure to a low amount of nitrate. Also, in this case, net uptake was equivalent to influx. In tungstate-treated plants preloaded with nitrate, both net uptake and influx were nil. In contrast to these observations, a clear discrepancy between net uptake and influx was observed when the plants were incubated at an initial nitrate concentration of approximately 5 micromolar, where net uptake is low and eventually ceases. It is concluded that plasmalemma nitrate transport is essentially unidirectional in plants performing net uptake at a concentration of 40 to 60 micromolar, and that transport is nil when internal nitrate sinks (vacuole, metabolism) are eliminated. The efflux component becomes increasingly important when the external concentration approaches the threshold value for net nitrate uptake (the nitrate compensation point) where considerable exchange between internal and external nitrate occurs.  相似文献   

12.
In the present study, we examined the effects of long- and short-term hypoxia on net uptake and transport of phosphorus to shoots of pond pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine, and the influence aerenchyma formation might have in maintenance of P uptake and transport. Seedlings were grown under aerobic (250 μM O2) or hypoxic (≤50 μM O2) solution conditions for 5.3 weeks in continuously flowing solution culture containing 100 μM P. Intact seedlings were then labeled with 32P for up to 24 h to determine how short- and long-term hypoxic solution conditions affected rates of unidirectional influx and the accumulation of 32P in roots and shoots. Seedlings in the long-term hypoxic treatment were grown for 5.3 weeks in hypoxic solution and also labeled in hypoxic uptake solution. The short-term hypoxic treatments included a 24-h hypoxic pretreatment followed by time in labeled hypoxic uptake solution for seedlings grown under aerobic or hypoxic conditions; in the latter case, diffusion of atmospheric O2 entry into stem and root collar lenticels was blocked, thus removing any influence that aerenchyma formation might have had on enhancing O2 concentrations of root tissue. Although unidirectional influx rates of 32P in roots of seedlings grown under long-term hypoxic conditions were 1.4 times those of aerobically grown seedlings, accumulation of 32P in roots was similar after 24 h in labeled uptake solution. These results suggest that 32P efflux was also higher under hypoxic conditions. Higher shoot/root fresh weight ratios and lower shoot P concentrations in seedlings grown under hypoxic solution conditions suggest that the “shoot P demand” per unit root should be high. Yet accumulation of 32P in shoots was reduced by 50% after 24 h in hypoxic uptake solution. Both short-term hypoxic treatments decreased accumulation of 32P in roots by more than 50%. Short-term hypoxia decreased shoot accumulation in seedlings grown under aerobic and hypoxic conditions by 84 and 50%. respectively. Short- and long-term hypoxic conditions increased the percentage of root 32P in the nucleic acid and chelated-P pools, resulting in a significantly smaller percentage of 32P in the soluble inorganic phosphate (pi) pool, the pool available for transport to the shoot. However, a reduction in pool size or in labeling of the pool available for transport cannot fully account for the large reduction in accumulation of 32P in shoots, particularly in the short-term hypoxic treatment of aerobically grown seedlings. Our results suggest that both influx and transport of 32P to shoots of pond pine seedlings are O2-dependent processes, and that the transport of 32P to shoots may be more sensitive to hypoxic solution conditions than influx at the cortical and epidermal plasmalemma, with aerenchyma formation supporting a substantial amount of both 32P uptake and transport.  相似文献   

13.
Translocation of nickel in xylem exudate of plants   总被引:3,自引:3,他引:0       下载免费PDF全文
Tiffin LO 《Plant physiology》1971,48(3):273-277
Topped plants of tomato (Lycopersicon esculentum), cucumber (Cucumis sativus), corn (Zea mays), carrot (Daucus carota), and peanut (Arachis hypogaea) were treated with 0.5 to 50 micromolar Ni (containing 63Ni) in nutrient solutions. Xylem exudate was collected for 10 hours or, in the case of corn, for 20 hours at 5-hour intervals. Electrophoresis of nutrient solution distributed all Ni cathodically as inorganic Ni2+. Low concentrations of Ni in tomato exudate migrated anodically, presumably bound to organic anion (carrier). However, this carrier became saturated at about 2 micromolar Ni in exudate, and excess Ni ran cathodically. Most of the Ni in cucumber, corn, carrot, and peanut exudate ran anodically, and its migration rate was identical for all exudates. Peanut root sap contained 14 to 735 micromolar Ni. The anodic Ni carriers in root sap and exudate appear identical. The carrier in root sap became saturated near 100 micromolar Ni, as shown by cathodic streaking of Ni exceeding that concentration. It appears that all five species translocate low concentrations of Ni in the same anionic form.  相似文献   

14.
The influence of nitrogen stress on net nitrate uptake resulting from concomitant 15NO3 influx and 14NO3 efflux was examined in two 12-day-old inbred lines of maize. Plants grown on 14NO3 were deprived of nitrogen for up to 72 hours prior to the 12th day and then exposed for 0.5 hour to 0.15 millimolar nitrate containing 98.7 atom% 15N. The nitrate concentration of the roots declined from approximately 100 to 5 micromolar per gram fresh weight during deprivation, and 14NO3 efflux was linearly related to root nitrate concentration. Influx of 15NO3 was suppressed in nitrogen-replete plants and increased with nitrogen deprivation up to 24 hours, indicating a dissipation of factors suppressing influx. Longer periods of nitrogen-deprivation resulted in a decline in 15NO3 influx from its maximal rate. The two inbreds differed significantly in the onset and extent of this decline, although their patterns during initial release from influx suppression were similar. Except for plants of high endogenous nitrogen status, net nitrate uptake was largely attributable to influx, and genetic variation in the regulation of this process is implied.  相似文献   

15.
A computer-controlled multichannel data acquisition system was employed to obtain continuous measurements of net nitrate or chlorate uptake by roots of intact barley plants (Hordeum vulgare cv Betzes) using nitrate-specific electrodes. Plants, previously grown in solutions maintained at 10 or 200 micromolar NO3 (low N or high N conditions, respectively), were provided with 200 micromolar NO3 or ClO3 during the uptake period. Initial rates of NO3 uptake were several times higher in low N plants than in high N plants. Within 10 min, uptake in the former plants declined to a new steady rate which was sustained for the remainder of the experiment. No such time-dependent changes were evident in the high N plants. Rates and patterns of net chlorate uptake exhibited almost identical dependence upon previous nitrate provision. NO3 (36ClO3) influx, by contrast, appeared to be independent of NO3 pretreatment prior to influx determination. Nitrate efflux, estimated by several different methods, was strongly correlated with internal nitrate concentration of the roots.  相似文献   

16.
Rengel Z 《Plant physiology》1990,93(3):1261-1267
Rhizotoxicity of Al is more pronounced in younger plants. Effects of Al on nutrient uptake by plants of different age are poorly understood. The depletion technique was used to monitor net Mg2+ uptake from nutrient solutions by intact 15- and 35-day-old plants of two ryegrass (Lolium multiflorum Lam.) cultivars. Lowering the pH from 6.0 to 4.2 decreased the maximum net ion influx without affecting Km. Aluminum at 6.6 micromolar Al3+ activity increased Km indicating competitive inhibition. The effects of pH and 6.6 micromolar Al3+ on net Mg2+ uptake were much larger in 15- than in 35-day-old plants. Aluminum at 26 micromolar Al3+ activity competitively inhibited net Mg2+ uptake by 35-day-old plants, while causing time- and external Mg2+ activity-dependent net Mg2+ efflux from 15-day-old plants. The equilibrium constant (Ki) of a reversible combination of postulated plasmalemma Mg2+ transporter and Al3+ was calculated to be 2 and 5 micromolar Al3+ activity for 15-day-old plants of Wilo and Gulf ryegrass, respectively, and 21 micromolar Al3+ activity for 35-day-old plants of both cultivars. The Al3+-mediated increase in Km was larger for 15-day-old plants of the Al-sensitive cultivar `Wilo' than of the more Al-tolerant cultivar `Gulf,' while Al3+ affected 35-day-old plants of both cultivars to the same extent.  相似文献   

17.
The frog ventricle in sucrose solution contracts for several hours at 25°C, and for as long as 24 hours at 5°G. The possibility that a fraction of the extracellular fluid remains outside of the excitable membrane was examined by measuring the efflux of tracers. The half-time for the efflux to sucrose solution at 25°C of C14 sucrose is about 1 minute, for Na24 is 6.5 minutes, and for Cl86 is 4 minutes. There is no evidence for the retention of an extracellular Na fraction. The Q10 for Na and Cl efflux is about 1.3. The half-time for K42 efflux is about 180 minutes; the Q10 is 1.7. The efflux rates of Na24, Cl36 and K42 to sucrose and to Ringer's solutions are quite similar. Ca45 efflux is only one-fifth as fast to sucrose solution as to Ringer's; the retention of Ca++ may be important for maintaining excitability in sucrose solution. P32 efflux is five times faster to sucrose solution than to Ringer's solution, and there is a similar increase in the rate of inosine loss to sucrose solution. The Q10 for efflux to sucrose solution is 2.2 for P32O4 and 2.4 for inosine. We suggest that energy metabolism is abnormal in ventricles in sucrose solution and that low temperature prolongs excitability by slowing the metabolic change.  相似文献   

18.
To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03–100 µmol P L-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, Imax, 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, Imax was negatively related to % P in shoots. Michaelis constants, Km, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, Cmin, where net influx equals zero, was found between 0.06 and 0.3 µmol L-1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles.It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of Imax, when internal P concentration decreases. Changes of Km and Cmin were of minor importance.  相似文献   

19.
The influx and efflux of organic acids across the root-soil interface were investigated in intact, sterile maize (Zea mays L.) roots under a variety of experimental conditions. Under nutrient-sufficient conditions the efflux of organic acids was shown to constitute < 1% of the total C lost across the root-soil interface. Under severe nutrient stress, however, the rates of malate and citrate efflux from the root increased 33 and 12 fold respectively. Influx experiments indicated that roots could not directly reabsorb citrate-Fe3+ or other metal complexes from solution. Influx of citrate was observed only at high external citrate concentration ( 1 mM) or from solutions with low ionic strengths. It was postulated that citrate influx is of little importance in a soil environment.  相似文献   

20.
Summary Wheat (Triticum vulgare L., cv. Blueboy) seedlings, grown with 0.25, 1.0 and 15 mM nitrate in complete nutrient solutions, were transferred 10 days after germination to 1.0 mM K15NO3 (99 A% 15N) plus 0.1 mM CaSO4 at pH 6.0. The solutions were replaced periodically over a 6-h period (5 mW cm-2; 23°). Changes in the [15N]- and [14N]nitrate in the solution were determined by nitrate reductase and mass-spectrometric procedures and potassium by flame photometry. Influx of [15N]nitrate was depressed in plants grown at 1.0 mM nitrate relative to those grown at 0.25 mM, but there was no appreciably difference in [14N]nitrate efflux. Prior growth at 15 mM further restricted [15N]nitrate influx which, together with a substantial increase in [14N]nitrate efflux, resulted in no net nitrate uptake during the course of the experiment. Efflux of [14N]nitrate occurred to solutions containing no nitrate but it was significantly enhanced upon exposure to [15N]nitrate in the external solution. Influx of [15N]nitrate was more restricted at 5°, relative to 23°, than was [14N]nitrate efflux. The nitrate concentrations of the root tissue immediately before exposure to the K15NO3 solutions did not give a precise indication of the subsequent [15N]nitrate influx rates nor of the [14N]nitrate efflux rates. Net K+ uptake was related to the magnitude of the net nitrate uptake, not to the initial K+ concentration in the roots. The data are interpreted as indicating that [15N]nitrate influx and [14N]nitrate efflux are largely independent processes, subject to different controls, and that net nitrate uptake provides the driving force for net potassium uptake.Paper No. 4884 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, NC, USA. This investigation was supported in part by the U.S. Energy Research and Development Administration, Contract No. AT-(40-1)-2410  相似文献   

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